1. Field of the Invention
The present invention relates to a display control apparatus and, more particularly, to a display control apparatus for a display device having a display element which uses, e.g., a ferroelectric liquid crystal as an operating medium for updating a display state and can hold an updated display state upon application or the like of an electric field.
2. Related Background Art
A display device is used as an information display means for achieving a visual information representing function in an information processing system or the like. A CRT display device (to be referred to as a CRT hereinafter) is generally used as such a display device.
Various information processing systems such as so-called personal computers are available in accordance with hardware, software, and signal transmission schemes. In this case, CRT display control apparatuses (CRTC) unique to various systems are used. Such CRTCs are exemplified by a VGA81 (available from IBM) as a VGA (Video Graphics Array) dedicated for an information processing system PC-AT (available from IBM) and an 86C911 (available from S3) as an SVGA (Super VGA) obtained such that an accelerator function for displaying predetermined images such as a circle and a rectangle is added to the VGA.
FIG. 1 is a block diagram showing an SVGA arrangement used in a CRTC.
When the host CPU of an information processing system partially rewrites a display memory window area in a host memory space, the rewritten display data is transferred to a VRAM 3 through a system bus 40 and a SVGA 1. The SVGA 1 generates a VRAM address on the basis of the address of the display memory window area and rewrites the display data in the VRAM 3 which is located at this VRAM address.
Meanwhile, the SVGA 1 accesses the VRAM 3 at the same period as the scan period of the CRT and sequentially reads out display data developed in the VRAM 3. The readout data are transferred to a RAMDAC 2. The RAMDAC 2 sequentially converts the input display data into R, G, and B analog signals and transfers the converted analog signals to a CRT 4. The SVGA used as the CRT display control apparatus functions to unconditionally transfer the display data at a predetermined period to the CRT.
In the above CRT display control, since the VRAM 3 comprises a dual port RAM, the VRAM 3 can independently perform an operation of writing display data in the VRAM to update the display information and an operation of reading out the display data from the VRAM. For this reason, the host CPU need not consider display timings and the like at all. Desired display data can be advantageously written at an arbitrary timing.
A CRT requires particularly a length in the direction of thickness of the display screen and has a large volume. It is difficult to obtain a compact CRT as a display device as a whole. This limits the degree of freedom of an information processing system using a CRT as a display. That is, the degrees of freedom in installation locations and portability are decreased.
A liquid crystal display (to be referred to as an LCD hereinafter) can be used as a display device which can compensate for the above drawbacks. More specifically, an LCD can achieve compactness (particularly, a low-profile configuration) of the display device as a whole. Of such LCDs, a display using a liquid crystal cell containing a ferroelectric liquid crystal (to be referred to as an FLC) is available. This display will be referred to as an FLCD hereinafter. One of the characteristic features of the FLCD lies in that the display state of the liquid crystal cell is preserved upon application of an electric field. That is, its liquid crystal cell is sufficiently thin, the elongated FLC molecules in the cell are aligned in the first or second stable state in accordance with an electric field application direction, and the aligned state of the molecules is maintained after the electric field is withdrawn. The FLCD has a memory function due to the above bistable operations of the FLC molecules. The details of the FLC and FLCD are described in U.S. Pat. No. 4,964,699.
Although the FLCD has the above memory function, it has a low FLC display updating speed. The FLCD cannot follow up with changes in display information which must be instantaneously updated. Such operations are exemplified by cursor movement, a character input, and scrolling.
In FLCDs having the above characteristics, various display drive modes which have originated from these characteristics or compensate for these characteristics are available. More specifically, in refresh driving for sequentially and continuously driving scan lines on the display screen as in a CRT and any other liquid crystal display, a relatively large time margin is available in its drive period. In addition to this refresh driving, partial rewrite driving for updating the display state of a part (line) subjected to a change on the display screen and interlace driving for interlacing and driving scan lines on the display screen are also proposed. The display information change speed can be increased by the partial rewrite driving or the interlace driving.
If display control of the FLCD having the above advantages can be performed using an existing CRT display controller, an information processing system using an FLCD as a display device can be arranged at a relatively low cost.